JP2004-133567A discloses an autonomously movable mobile body including a camera configured to be capable of capturing images of a ceiling surface and a position detector configured to process the image data captured by the camera to detect a position of the mobile body, wherein the position detector stores beforehand reference image information including ceiling images captured at reference positions whose position information is known, and the mobile body captures an image of the ceiling with a camera and detects the position of the mobile body based on the captured ceiling image and the reference image information by performing image matching.
In the mobile body disclosed in JP2004-133567A, two cameras spaced apart from each other are used to capture stereo images of the ceiling to detect a distance from the mobile body to each object included in the captured images. The detected distance to each object is compared to the distance to the ceiling (the distance to the ceiling is given beforehand) to extract only portions (or pixels) of each captured image corresponding to the ceiling or a part near the ceiling (or to mask the pixels corresponding to objects other than the ceiling, such as a wall, a door, a desk, a cabinet, etc.).
In a case where the mobile body is used in a large facility, such as an airport, each ceiling image captured by the mobile body covers only a part of the ceiling of the operation area of the mobile body in the facility, and it may be desired to combine the ceiling images captured at various positions to form a mosaic image of the ceiling. Such a mosaic ceiling image should preferably have a uniform scale over the entire region thereof so that calculation of a distance in the mosaic ceiling image is easy. In the present disclosure, a mosaic ceiling image having a substantially uniform scale over the entire region thereof is referred to as a ceiling panorama map or, simply, a ceiling map. The ceiling map may be used by the mobile body that was used to build the ceiling map or another mobile body to detect or estimate the position thereof (or to localize the mobile body). However, the height of the ceiling from the floor (and hence from the mobile body) may differ from position to position in the facility, and therefore, the captured ceiling images, which typically have the same resolution (e.g., 1936×1216), may have different scales, where a “scale” can be represented by a number of pixels in the image corresponding to a unit length (e.g., one meter). Therefore, in order to build a ceiling map, it is necessary to adjust the sizes of the captured ceiling images according to the respective scales such that the adjusted images have a substantially same scale when combining them to form the ceiling map. However, the scales of the captured ceiling images are often unknown because the height of the ceiling from the floor at each image-capturing position is often unknown.
In order to determine the distance to the ceiling and to thereby determine the scale of each of the captured ceiling images, it may be conceived to capture the images as stereo images by use of two cameras, as in JP2004-133567A. However, using two cameras would make the system undesirably complicated.